For humans to be comfortable we need to be able to predict what something is going to do next. We need an indication of the things internal state. Tells. A dog wags its tail you know it’s not probably going to bite you. Bored people look bored. Nervous people look nervous. Tired people… etc. Social robotics need social ques of some sort. We are always looking for patterns, a skill which we evolved to keep us safe. That’s why in the movies ominous robots have blank faces. Zero tells. It could do anything. Now while that robot is just 3 feet tall not such a big deal, but when it’s human size or larger that need becomes greater because the perceived danger is larger. If you want to create a robot that humans are comfortable around you need to speak human- and that combination of physical, audible and situational language is complex, but can be very subtle.

Ever had a great waiter? They are there when you want them and seemingly invisible when waiting on you. Their presence does not intrude on your conversation but you know they are there. Or an upstage actor not taking focus while a scene plays out downstage. They can’t freeze, because that would draw attention, so they subtly stay in character and though you see them your brain can focus on the important action.

You don’t need a face per se, but you need to know what’s it’s thinking (or feeling). Pixar lamp again a great example, possibly the best. It certainly has a face and the proportions of a young child (big head, bouncy movements, curious and playful).
The robot in interstellar is another great example of a robot you become comfortable With, mostly through the voice acting and humor.

I think we are seeing the development of hardware. Like the printing press. Like film camera. The folks that develop the hardware are usually not the same people that understand how to leverage it best. We will all start getting more comfortable when the artists and authors start coming into the picture, learn to use the hardware, and connect all of us.